1. Field of the Invention
This invention relates to a structure and method for searching computer data bases in order to locate and retrieve textual information.
2. Description of the Prior Art
Prior art text comparators for searching a computer data base are known. Structures for carrying out such techniques (such structures are herein called "textual comparison systems") are used, for example, by Lockheed Dialog Information Retrieval Service, the United States Government "Flite" service, "Lexis", and others.
Such prior art textual comparison systems are software oriented in that a portion of the information stored in the computer (called a "data base") must be loaded into the computer working memory from a mass memory storage device (typically a magnetic disk). The portion of the data base within the working memory of the computer is scanned by the computer, as controlled by software instructions, in order to determine if any portion of the data base stored in the computer working memory matches the desired text. Typically the textual material comprising the data base is stored by using a set of standard data base characters such as the well-known and commonly used American National Standard Code for Information Interchange ("ASCII"). The ASCII characters and their binary and hexidecimal representations are shown in Table 1. Thus, such prior art software-oriented text comparators are rather slow in that the computer must control the transfer of sequential portions of the data base from a large storage media, such as a disk, to the computer memory, and the computer must then utilize an iterative process in order to determine whether the desired text is contained within that portion of the data base which has been transferred to the computer memory. Because the computer itself is performing the search, such prior art searching techniques are rather slow, and consequently expensive due to the large amount of computer time required to perform a search.
Another prior art comparator system is described in U.S. Pat. No. 4,152,762 issued May 1, 1979 to Bird et al. Bird et al describe a method and structure for text comparison which is rather complex and requires each desired textual word or phrase to be stored in octal format in one of a plurality of "key memories". In addition, the Bird structure requires the use of additional memories, including a "pointer memory" and a "hash memory", as well as a wide variety of other subcircuits. Thus, the Bird structure is rather complex.